In systems wherein one- or two-way communication is performed between nodes, e.g. units, apparatus, etc. by means of wireless transmission, for example radio frequency transmission, it is of importance that a node, e.g. an apparatus, will be able to receive a transmitted signal, e.g. a message, a dataframe etc. where such a signal has been transmitted from e.g. another node in the system. Normally, this is achieved by letting the node in questions be in a receive mode constantly, e.g. by having all necessary circuits and in particular the radio frequency circuits be active continuously. This will of course contribute significantly to the total power consumption of the node in question.
Therefore, there is a need to design and/or operate such systems and nodes utilized in such systems in a manner, whereby power consumption is reduced. This need is enhanced owing to the fact that many of the nodes involved in such systems rely on power supply systems where maximum power, rating and/or capacity are limited, such as battery power supply.
Further, the need to be able to establish a communication to or between nodes without time delay or without an unacceptable time delay, e.g. unacceptable in the specific circumstances, must be kept in mind when designing such systems and/or methods for operation.
WO 00/28776 A1 relates to a method for operating a transmitter and receiver units in a control system. In order to reduce the power consumption of the receiver units one or each of these is activated at a specific interval in time, which is synchronized with the transmission cycle of the transmission unit. The transmission unit transmits in intervals, e.g. 300 msec, with a predetermined and fixed distance, e.g. four minutes, between the transmission intervals. If a transmission unit transmits to a number of receiver units, the distances between the transmit intervals corresponding to the respective receiver units may differ from each other, but the distance between intervals for a specific receiver unit will be fixed. The regular signals transmitted from the transmitter to the receiver unit does not comprise information concerning the distance between intervals, but a special synchronization signal containing information regarding the distance is transmitted by the transmitter halfway between intervals. Thus, if a receiver unit falls out of synchronicity it will stay active until such a synchronization signal is received. The receiver will then be able to determine the next interval where it must be active, i.e., as half of the fixed distance, e.g. two minutes when the fixed distance is four minutes, measured from the time of reception of the synchronization signal.
Thus it is apparent that the prior art of WO 00/28776 A1 is related only to energy-savings for the receiver unit(s) and not the transmitter unit. Further, the transmitter must be arranged to transmit separate synchronization signals during every period and at a specific time, i.e. exactly between the regular transmit intervals. Further, it is noted that the transmission frequency as well as the distance between transmittals is fixed for this prior art system. WO 99/46745 A2 relates to a method of transmitting data from several first stations to a second station. According to the method the transmissions are arranged in time windows that are repeated consecutively. In these time windows slots are arranged, i.e. a first and a second slot for transmission of a synchronization message and a selection message, respectively, from the second station. The synchronization message serves to synchronize clocks of the first stations with the clock of the second station. The selection message serves to transmit identification of the first stations and corresponding slots for transmittals of data from these. These slots for transmittals of data from the first stations follow immediately after the selection message. In these slots the respective first stations transmit data to the second station in their respective slots. Each first station thus has to be active during the first and the second slot for transmission of a synchronization message and a selection message. Hereafter each first station may switch off its receiver, while its transmitter needs to be active at the allotted response time slot. After having transmitted data to the second station at this time slot from the first station in question this station may switch off its transmitter again. When a new time window begins immediately after the end of the time window in question, the receivers of the first stations need to be active again. According to another embodiment further slots are included wherein command message slots are included between the selection message slot and the response slots. In these slots information regarding the response time slots are given and in addition further instructions may be given to the first stations.
Thus, this prior art system requires that general synchronization messages are transmitted for each cycle and thus requires that the receivers of the first stations are active/switched on while the synchronization messages are transmitted. Further, these receivers need to be switched on for the following selection message slot as well. Thus all receivers must be active also in cases where no information has to be transmitted to the first station in question or no response is needed from the first station in question. Further it is noted that the second station must always be activated. Finally, because of the cyclic nature of the system, a certain power drain will be present for transmission and reception of synchronization and selection messages, even in cases where activity is not required or is restricted.
EP 0 529 269 A2 relates to a method and an apparatus for controlling a wireless link adapter of a battery powered computer in order to conserve battery power. A multi-access protocol is used in which time is divided into fixed-length frames, and these frames are divided into slots. A frame, which is repeated cyclically, is divided into three subframes, one for transmission of data packets from a base station to mobile units, and two for allowing transmission the other way. Each subframe has a header wherein the mobile units are allotted a slot each. Thus, in the first subframe, the receiver of a mobile unit need only to be active in the allotted slot and correspondingly for the transmitter in the following two subframes. Thus, the drain on the batter power is reduced. However, as mentioned above, synchronization must be performed repeatedly and the receiver must be turned on for each header even in cases where there is no need to transmit to or from the mobile unit in question.
EP 0 748 085 A1 relates to a system comprising a number of battery powered mobile units, one of which is designated as a master unit, and the rest are designated as slave units. Communication between the master unit and the slave units, i.e. communication in order to assess whether the respective slave units are still present, takes place cyclically with periods of a considerably and fixed length, e.g. a time window of one hour divided into sub-windows of 20 minutes length. The slave units have each been allocated a slot and need only be active in their respective slot, whereby battery drain is reduced for slave units. However, as mentioned above, the transmitted information is limited to information regarding presence or not. Further all slave units need to be activated every period, and also due to the fixed length of period and the considerable length of the cycle, this prior art system has limited application possibilities.